Isomers are molecules or molecular compounds that are similar in that they have the same molecular formula, however have different arrangements of the atoms or groups of atoms (functional groups) involved.

The word Isomer comes from the Greek words, Isos meaning equal and meros, meaning part, or to share.

As an example, glucose, while maintaining its basic structure, can arrange its atoms or functional groups in a number of different, spatial arrangements, forming a number of isomers of glucose, with different properties.

Stereoisomerism - Optical Isomers

Optical isomers are one form of stereoisomerism.

Optical isomers are molecules that are mirror images of each other.

If you were to construct models of them, they would not be able to be super imposed on each other in the same way that your right and left hand can not be superimposed on each other.

These types of isomers were found in many cases to affect plane-polarized light by rotating it either to the right or left and hence termed optically active molecules, and it is for this reason that they are called optical isomers.

The molecule that rotates the plane-polarized light to the right is called dextro-rotary (D-isomer, Latin dexter = right) and to the left, laevo-rotary (L-isomer, Latin laevus = left).

Another convention using R (from Latin rectus for right) and S (from Latin sinister for left) for D and L respectively is also used.

When only two optical isomers are possible they are called enatiomers.

That such a small difference in a molecule can be of great significant is indicated by the different properties the optically active molecules of thalidomide and LSD molecules can have. (more).

Stereoisomerism - Conformational Isomers

A conformational isomer is another form of stereoisomerism.

Note the position of the hydrogen (H) and hydroxyl group (OH) at carbon one (1C) of the glucose molecule.

These positions can be reversed as indicated below, with the molecules being referred to as Alpha () and Beta () isomers, depending on the positions of their rotatable functional groups.